For every passage, \(P_\text{selection}(i)\)  is the score that this passage contains the answer and for every span \(P_{\text{start},i}(s) \cdot P_{\text{end},i}(t)\)  is the score of a single span (\(s\)  to \(t\) ) in a passage.
In this specific model, the aggregator simply passes on all the retrieved passages, though some models simply concatenate all passages and pass it to the model as single string input [1]}.
